1. Field of the Invention
This invention relates to a pneumatic booster for assisting in operating a master cylinder of brake and clutch systems of a vehicle or the like.
2. Prior Art
One conventional booster under consideration comprises a hollow body having front and rear walls; a movable wall mounted in the hollow body to divide the interior of the body to a front low pressure chamber and a rear high pressure chamber, the movable wall being movable relative to the body along an axis thereof, the movable wall having a rear portion slidably extending through the rear wall of the body, and the movable wall having an internal bore extending along the axis of the body and passing through the rear portion; an elongated input member extending into the internal bore from its rear end and being movable forwardly; a plunger mounted in the internal bore and connected to a front end of the input member for moving therewith, the plunger having a first valve seat at its rear end, and a second valve seat being formed on the surface of the internal bore; a valve member mounted within the internal bore for sealing engagement with the first and second valve seats; an output member received in a front end of the internal bore; and load-transmitting means interposed between the output member and the plunger for engagement therewith. In accordance with the forward movement of the input member relative to the movable wall, the positions of the first and second valve seats relative to the valve member are so controlled as to release, maintain and develop a pressure differential between the low and high pressure chambers. In an inoperative condition of the booster, the valve member is spaced considerably from the second valve seat. Upon forward movement of the input member for initiating the operation of the booster, the valve member is brought out of engagement with the first valve seat and brought into engagement with the second valve seat to bring the booster into its pressure differential-developing condition in which the pressure differential develops between the low and high pressure chambers. The above-mentioned spacing of the valve member from the second valve seat in the inoperative condition of the booster retards the initiation of the operation of the booster. In order to improve this retarding, it has been proposed to hold the valve member in engagement with the second valve seat in the inoperative condition of the booster under the influence of a return spring.
One conventional means for improving this retarding as shown in U.S. Pat. No. 4,398,449 comprises a rod extending through the interior of the hollow body axially thereof and fixed to the front and rear walls thereof, and a generally rectangular plate disposed perpendicularly to the rod, the plate having at its outer end an aperture slidably fitted on the rod and having at its inner end a slot received in a peripheral groove formed in the plunger in such a manner as to allow an axial movement of the plunger relative to the plate, the groove having a pair of axially opposed front and rear walls. The rod has a shoulder adjacent to the rear wall of the hollow body with which shoulder the plate is engageable to limit its rearward movement. With this construction, upon rearward movement of the input member, the outer end of the plate is brought into engagement with the shoulder under the bias of the return spring, and the front wall of the groove of the plunger is brought into engagement with the inner end of the plate to limit the rearward movement of the plunger so that the movable wall is allowed to further move rearwardly relative to the plunger to bring the valve member into engagement with the second valve seat in the inoperative condition of the booster. However, as described above, the plate engages the plunger at its inner end and engages the rod at its outer end. Therefore, the plate tends to become displaced from the plane perpendicular to the axis of the plunger, so that the plunger is also inclined relative to the axis of the internal bore. As a result, the first valve seat on the plunger is partly spaced from the valve member during the movement of the plunger to develop an undesirable pressure leakage. In addition, the inclination of the plunger causes damage to the surface of the internal bore during its movement.